The head-to-tail and head-to-head condensation reactions in the terpene biosynthetic pathway are key steps in biosynthesis of cholesterol, and are catalyzed by the enzymes prenyl transferase and squalene synthetase. We are studying the mechanisms of the enzyme catalyzed condensation reactions with substrate analogues whose reactivities differ from those of the natural substrates in a predictable manner. The enzymes will be obtained from avian or porcine liver and will be purified to remove other terpene-related catalytic activity. Substrate analogues containing fluorine or sulfur will be prepared. Kinetic and product studies will be carried out with the substrate analogues and the appropriate condensation enzyme. We will use the analogues to study the role feedback of inhibition by C5 to C30 metabolites for regulating biosynthesis of cholesterol.